N-Acetyl-β-hexosaminidase (HEX), a member of lysosomal hydrolases, catalyzes hydrolysis of terminal, non-reducing N-acetyl-β-D-glucosamine (GlcNAc) and N-acetyl-β-D-galactosamine (GalNAc) residues in glycoproteins, gangliosides, and glycosaminoglycans (GAGs).
HEX, released by chondrocytes into the extracellular compartment, promotes cartilage matrix degradation. Osteoarthritis patients have increased HEX activity in synovial fluid. See Steinberg et al., Biochim. Biophys. Acta 1983 757(1): 47. It has been shown that HEX inhibitors prevent or even reverse cartilage matrix degradation and therefore can be used in treating osteoarthritis. See, e.g., Liu et al. Chem. Biol. 2001, 8, 701-11; and Amorelli et al. Bioorg. Med. Chem. Lett. 2008, 18, 2944-2947.
HEX is also associated to lysosomal storage disorders. For example, Tay-Sachs disease and Sandhoff disease are caused by inherited missense mutations. It has been found that these missense mutations do not affect the active site of the enzyme but, rather, inhibit its ability to obtain or retain its native folding in the endoplasmic reticulum, resulting in accelerated degradation and decreased transport to lysosome. See, e.g., Tropak et al., J Biol Chem. 2004; 279(14):13478-87. HEX inhibitors, acting as pharmacological chaperones by binding to the active site, enhance the HEX stability and increase their transport to lysosome. See Tropak et al., Chem. Biol. 2007; 14(2): 153-64. They can be used to treat Tay-Sachs and possibly Sandhoff diseases.
A major barrier to clinical use of HEX inhibitors is that they may also inhibit O-GlcNAcase, an enzyme sharing a common catalytic mechanism with HEX, thereby causing side effects. See, e.g., Vocadlo et al., J. Biol. Chem. 2005, 280, 25313-25322 and Vocadlo et al., Nat. Struct. Mol. Biol. 2006, 13, 365-71. There remains a great need for discovering a drug that selectively inhibits HEX over O-GlcNAcase.